Stuck Transactions

Hello, I am repeatedly getting transactions that are stuck, meaning they will say "0/unconfirmed, in memory pool". However, several blocks have gone by, each with only a few transactions from other people, and mine are still unconfirmed. Any ideas?

This happened more frequently after upgrading to the latest release v0.14.5, but is also occurring with v0.14.3. I am using the "sendtoaddress" over RPC from the command line. It works most of the time, but then the transactions stop going through.